Assuming you're talking about a Boot Camp partition, I certainly hope not. I didn't see anything on the Apple site specifically addressing existing BC installs, but I have to believe that it should transition with no issues.
i am going to wipe my entire HDD, since i just got my mbp last week and i want to install everything from scratch. i just installed bootcamp to try it out. i also don't want the partition to be so big.
i'm just going to format the whole drive, and see if i can get leopard to take up 5gb or less on my HDD instead of the 12GB Tiger is taking up right now.